Asymmetric wireless system

ABSTRACT

A method and apparatus is disclosed herein for sensor node and access point communication. In one embodiment, the method comprises embedding, by a sensor node, data from one or more sensor readings in a field of a first message, the field being designated as part of a standardized protocol to send information other than the data sensor readings; determining without the use of a receiver, by the sensor node, whether a wireless communication channel is at a predetermined quality level to send the first message; and broadcasting the first message wirelessly to a component according to the standardized protocol.

FIELD OF THE INVENTION

Embodiments of the present invention relate to the field ofradio-frequency (RF) communication; more particularly, embodiments ofthe present invention relate to embedding data, such as data from sensorreadings, in messages and sending those messages wirelessly to otherdevices.

BACKGROUND OF THE INVENTION

Wireless sensors often transmit data to servers, directly or indirectly.There are many available wireless transmission solutions in the marketto perform the wireless transmission of data from these wirelesssensors, including Wi-Fi, Bluetooth, Zigbee, etc. These all share onesignificant problem: energy consumption. For example, data from a vendorof the lowest power Wi-Fi chip on the market shows that 220 mJ arerequired to send a single User Datagram Protocol (UDP) packet. This willdischarge a standard AA battery very quickly, depending on the dutycycle used. This is a significant problem for current products since thelabor cost to replace batteries can quickly exceed the applicationbenefit to the customer.

A cost issue related to the use of some current wireless transmissiontechniques involves the wireless receiver infrastructure. Many of thealternatives to Wi-Fi such as, for example, Bluetooth or Zigbee, requiredevices that bridge their protocols to Wi-Fi, in order to reach theinternet “cloud”. This poses not only a cost issue at installation timebut also a maintenance headache for application areas (e.g., sensors insupermarkets) that are extremely cost sensitive and where it's almostimpossible to allocate any labor to sensor maintenance.

As discussed above, Wi-Fi has been used for communication with wirelesssensors. FIG. 9 illustrates messages (or “frames” in Wi-Fi terminology)exchanged between a wireless sensor, which operates as a Wi-Fi client,and an access point which operates as a Wi-Fi host. Referring to FIG. 9,when a client enters a new Wi-Fi environment, the user starts the systemand the client broadcasts a probe request frame (PRF) (901). Accesspoints that receive the PRF respond and the client begins an exchange ofmessages that sets up a two-way channel with one of the access points.Data is exchanged (902) after a long sequence of messages has beenexchanged. The energy to reach this point is 220 mJ with astate-of-the-art Wi-Fi chip (Atheros AP4100).

Probe request frames are typically received only when a connection isestablished and they contain logical information such as the encryptioncapabilities of the client that is used to configure aspects of thehardware that support data exchange.

SUMMARY OF THE INVENTION

A method and apparatus is disclosed herein for sensor node and accesspoint communication. In one embodiment, the method comprises embedding,by a sensor node, data from one or more sensor readings in a field of afirst message, the field being designated as part of a standardizedprotocol to send information other than the data sensor readings;determining without the use of a receiver, by the sensor node, whether awireless communication channel is at a predetermined quality level tosend the first message; and broadcasting the first message wirelessly toa component according to the standardized protocol.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood more fully from the detaileddescription given below and from the accompanying drawings of variousembodiments of the invention, which, however, should not be taken tolimit the invention to the specific embodiments, but are for explanationand understanding only.

FIG. 1A is a block diagram of an embodiment of a wireless sensor node.

FIG. 1B is a block diagram of another embodiment of a wireless sensornode.

FIG. 2 is a flow diagram of one embodiment of a process for generatingand sending messages using a sensor node.

FIG. 3 illustrates one embodiment of an asymmetric Wi-Fi communicationsystem.

FIG. 4 illustrates one embodiment of an access point.

FIG. 5 is a flow diagram of a process for generating and sending amessage that includes sensor readings data from a sensor node.

FIG. 6 illustrates a comparator-based low voltage detector.

FIG. 7 illustrates an opamp-based low voltage detector.

FIGS. 8A-8C illustrate techniques used by monitoring units to determinewhether a wireless communication channel is clear enough to send a proberequest message.

FIG. 9 illustrates prior art message exchange between a wireless sensorand an access point.

FIG. 10 is a block diagram of one embodiment of a transmitter.

FIG. 11 is a flow diagram of an alternative embodiment of a process forgenerating and sending messages using a sensor node.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

A technique for one-way wireless communication by sensor nodes isdisclosed. In one embodiment, data from sensor readings is embedded inprobe request messages that Wi-Fi clients typically transmit when theycome up on the air. The probe request messages are created by wirelesssensor nodes, which send them to access points. The access pointsprocess the probe request messages. In one embodiment, this processingoccurs at the processor level of the access point rather than infirmware where the processor runs a rudimentary operating system thatcan intercept probe request frames, extract the data from sensorreadings, and send the extracted sensor readings data via a transportlayer UDP packet containing the sensor data to a remote destination(e.g., a destination remote with respect to the access point and thewireless sensor node). In one embodiment, the destination is an IPnetwork destination. In such a case, this provides a complete solutionfor transmitting sensor readings to any destination on the Internet.

Thus, part of the Wi-Fi protocol is repurposed to provide a method forsending data from sensors at any time (not just when they power up) toan existing Wi-Fi infrastructure. This is a huge commercial advantage incomparison to most other techniques for communication with wirelesssensors that require a separate infrastructure. In one embodiment, thistechnique also does not require an acknowledgment from the receiver.This is beneficial in that the hardware may be significantly reducedsince the receiver is not included, nor is needed. Also, in oneembodiment, Wi-Fi access points are retrofitted with software thatconverts probe requests containing sensor data into standard TCP/IPmessages (e.g., UDP packets) that can be sent to specified destinations.

In the following description, numerous details are set forth to providea more thorough explanation of the present invention. It will beapparent, however, to one skilled in the art, that the present inventionmay be practiced without these specific details. In other instances,well-known structures and devices are shown in block diagram form,rather than in detail, in order to avoid obscuring the presentinvention.

Some portions of the detailed descriptions which follow are presented interms of algorithms and symbolic representations of operations on databits within a computer memory. These algorithmic descriptions andrepresentations are the means used by those skilled in the dataprocessing arts to most effectively convey the substance of their workto others skilled in the art. An algorithm is here, and generally,conceived to be a self-consistent sequence of steps leading to a desiredresult. The steps are those requiring physical manipulations of physicalquantities. Usually, though not necessarily, these quantities take theform of electrical or magnetic signals capable of being stored,transferred, combined, compared, and otherwise manipulated. It hasproven convenient at times, principally for reasons of common usage, torefer to these signals as bits, values, elements, symbols, characters,terms, numbers, or the like.

It should be borne in mind, however, that all of these and similar termsare to be associated with the appropriate physical quantities and aremerely convenient labels applied to these quantities. Unlessspecifically stated otherwise as apparent from the following discussion,it is appreciated that throughout the description, discussions utilizingterms such as “processing” or “computing” or “calculating” or“determining” or “displaying” or the like, refer to the action andprocesses of a computer system, or similar electronic computing device,that manipulates and transforms data represented as physical(electronic) quantities within the computer system's registers andmemories into other data similarly represented as physical quantitieswithin the computer system memories or registers or other suchinformation storage, transmission or display devices.

The present invention also relates to apparatus for performing theoperations herein. This apparatus may be specially constructed for therequired purposes, or it may comprise a general purpose computerselectively activated or reconfigured by a computer program stored inthe computer. Such a computer program may be stored in a computerreadable storage medium, such as, but is not limited to, any type ofdisk including floppy disks, optical disks, CD-ROMs, andmagnetic-optical disks, read-only memories (ROMs), random accessmemories (RAMs), EPROMs, EEPROMs, magnetic or optical cards, or any typeof media suitable for storing electronic instructions, and each coupledto a computer system bus.

The algorithms and displays presented herein are not inherently relatedto any particular computer or other apparatus. Various general purposesystems may be used with programs in accordance with the teachingsherein, or it may prove convenient to construct more specializedapparatus to perform the required method steps. The required structurefor a variety of these systems will appear from the description below.In addition, the present invention is not described with reference toany particular programming language. It will be appreciated that avariety of programming languages may be used to implement the teachingsof the invention as described herein.

A machine-readable medium includes any mechanism for storing ortransmitting information in a form readable by a machine (e.g., acomputer). For example, a machine-readable medium includes read onlymemory (“ROM”); random access memory (“RAM”); magnetic disk storagemedia; optical storage media; flash memory devices; etc.

A Wi-Fi Communication System with a Sensor Node

One embodiment of a Wi-Fi wireless communication system is described. Inthe Wi-Fi communication system, communications occur at 2.4 GHz or 5.8GHz. Note that in alternative embodiments, communications in thewireless communication system occur at other radio frequencies.

In one embodiment, the Wi-Fi communication system is used as part of anintelligent sensor network having one or more sensor nodes (e.g., tags).In one embodiment, the sensor nodes harvest and store energy (e.g.,photovoltaic, thermal, vibrational, etc.), collect and process sensordata, and communicate with other devices (e.g., Wi-Fi devices) using acommunication standard (e.g., Wi-Fi, Zigbee, Bluetooth, Bluetooth LowEnergy), or even proprietary interfaces. In one embodiment in which thesensor nodes communicate via Wi-Fi, the sensor node communicates withanother Wi-Fi device by sending standard UDP packets. In anotherembodiment in which the sensor tags ultimately communicate via Wi-Fi,the sensor node communicates using a proprietary communication protocol(a Wi-Fi bridge is used in this case).

Note that although the sensor nodes are described herein as beinglow-power sensor nodes that harvest and store energy, the presentinvention is not limited to the use of such sensor nodes and thetechniques described herein are applicable to various sensor nodeconfigurations, including, but not limited to those that containwireless communication chips (e.g., Wi-Fi chips).

In one embodiment, the intelligent sensor network includes Wi-Fi sensornodes (e.g., tags). FIG. 1A illustrates one embodiment of a sensor node120 (e.g., sensor tag) that communicates over Wi-Fi. Referring to FIG.1A, a sensor 121 generates sensor data and sends the sensor data to aprocessor 122. In response to the sensor data, processor 122 generatesone or more probe request messages and sends those probe requestmessages to analog transmitter 123, which transmits the probe requestmessages using antenna 124.

FIG. 1B illustrates another embodiment of a sensor node 100 (e.g.,sensor tag) that communicates over Wi-Fi. Referring to FIG. 1B, antenna101 is coupled to switch 103. The impedance matching circuits 102 a-c onthe outputs of switch 103 may comprise a passive network of componentsthat improves energy transfer from a source impedance to a loadimpedance. In one embodiment, impedance matching circuits 102 a-c are LCcircuits (i.e., a circuit with an inductor and a capacitor). Switch 103has a terminal coupled to RF transmitter 105 via impedance matchingcircuit 102 a. RF transmitter 105 transmits or broadcasts informationsuch as, for example, data from sensor readings wirelessly using antenna101. In one embodiment, RF transmitter 105 is an 802.11 Wi-Fitransmitter. RF transmitter 105 is coupled to microprocessor 106 and isresponsive to one or more control signals from microprocessor 106 totransmit information.

Another terminal of switch 103 is also coupled to energy harvesting andstorage circuitry 104 via impedance matching circuit 102 b. Energyharvesting and storage circuitry 104 is used to provide power totransmitter 105, microprocessor 106, monitoring unit 110, and sensors111. In one embodiment, energy harvesting and storage circuitry 104includes an energy harvesting unit and a storage unit. Energy harvestingand storage circuitry 104 receives energy via antenna 101 through switch103 and impedance matching circuit 102 b during energy harvesting andthe energy harvested is stored in an energy storage device. Theharvesting circuitry may include a diode based rectifier for convertingincoming RF energy to a DC voltage. In some embodiments, the diode basedrectifier may include Schottky diodes such as those manufactured byAvago Technologies Inc. The harvesting circuits may also include energymanagement functions based on discrete implementations known to thosefamiliar with the state of the art, or they could use parts such as theMAX17710 of Maxim Integrated™ or the LTC3108 of Linear Technology. Thestorage unit can be a capacitor, super-capacitor, or any type ofrechargeable battery technology such as, for example, an Eneloopbattery. Source energy for the charger could also be photovoltaic,thermal, or vibrational. Battery chargers for this type of energyharvesting are well-known to those familiar with the art. The sensornode may comprise a tag that includes tag material. The tag material canbe a standard printed circuit board, or a flexible tag printed on filmsuch as modern standard RFID tags.

Sensors 111 include one or more sensors that sense data and providesensed data to microprocessor 106. In one embodiment, sensors 111comprise one or more temperature, pressure, humidity, gas composition,image, and position sensors. In one embodiment, sensor node 100 spendsmost of its time asleep and wakes up to enable sensors 111 to sense dataand to transmit that data to locations remote to sensor node 100. Thereare a number of well-known techniques (e.g., interrupt-based techniques)that can be used to wake up sensor node 100 at different times (e.g.,pre-determined intervals) to take sensor readings.

In one embodiment, in response to one of sensors 111 sensing data, thesensor signals (e.g., generates an interrupt) to microprocessor 106 towake-up microprocessor 106 so that the sensed data can be stored on thenode (in RAM or ROM internal to microprocessor 106 such as memory 106 aor external to microprocessor 106 such as memory 112. Memory 106 couldalso be dual-ported in which case the sensor (111) could write intomemory while microprocessor 106 is asleep), so that it can be uploadedto the network (via wireless communication with another RF device thatis proximate to it) at a later time. In one embodiment, sensors 111interrupt microprocessor 106 only when their sense outputs changesignificantly enough to desire microprocessor 106 to wake up and capturethe new condition prior to going back to sleep. Finally, a sensedsituation might be significant enough (such as an alarm alert) that oneof sensors 111 wakes microprocessor 106 up for a communications event,in addition to a storage event.

Microprocessor 106 acts as a controller for sensor node 100. In oneembodiment, microprocessor 106 generates messages to be transmittedwirelessly, via transmitter 105 and antenna 101. In one embodiment,microprocessor 106 creates at least some messages by embedding data fromone or more sensor readings from sensors 111 in a field of the message,where the field is designated as part of a standardized protocol (e.g.,Wi-Fi) to send information other than the data sensor readings. In oneembodiment, the message is a Wi-Fi probe request message and the datasensor readings from sensors are embedded in a field used to specifyvendor information. In one embodiment, the field is the Vendor SpecificInformation as set forth in 802.11.

Microprocessor 106 sends the messages to transmitter 105 fortransmission to another wireless component (e.g., an access point, abase station, etc.). In one embodiment, microprocessor 106 sends themessages to transmitter 105 for transmission when notified that thewireless communication channel is at or above a predetermined qualitylevel.

Another terminal of switch 103 is also coupled to monitoring unit 110via impedance matching circuit 102 c. Monitoring unit 110 determineswhether a wireless communication channel that is going to be used totransmit data from sensor readings made by sensors 111 is at or abovethe predetermined quality level. In one embodiment, monitoring unit 110performs this by monitoring incoming radio frequency (RF) signalswithout the use of a wireless communication receiver. In other words,the determination is made without using a wireless communicationreceiver. In one embodiment, the determination is made by monitoringincoming radio frequency (RF) signals using an envelope detector (e.g. alogamp). In another embodiment, the determination is made by convertingincoming radio frequency (RF) signals to a voltage and comparing thevoltage to a threshold voltage level. The voltage conversion may beperformed by an RF power detector or a RF rectifier. These andadditional techniques are described in greater detail below.

When monitoring unit 110 determines the wireless communication channelis at or above the predetermined quality level (e.g., there is nocurrent wireless communication in progress), monitoring unit 110notifies microprocessor 106, via a signal or other indication, whichcauses microprocessor 106 to signal and/or otherwise control transmitter105 to broadcast the message containing the sensor readings.

FIG. 2 is a flow diagram of one embodiment of a process for generatingand sending messages using a sensor node. The process is performed byprocessing logic that may comprise hardware (circuitry, dedicated logic,etc.), software (such as is run on a general purpose computer system ora dedicated machine), or a combination of both.

Referring to FIG. 2, the process begins by processing logic embeddingdata from one or more sensor readings in a field of a first message(e.g., a probe request message) where the field is designated as part ofa standardized protocol (e.g., Wi-Fi) (processing block 201). Processinglogic determines whether a wireless communication channel is at apredetermined quality level to send the first message without the use ofa receiver (processing block 202) and then broadcasts or otherwisetransmits the first message wirelessly to a component according to thestandardized protocol (processing block 203).

Although not shown, sensor node of FIG. 1A may also include a monitoringunit such as monitoring unit 110 of FIG. 1B.

An Asymmetric Wi-Fi System Arrangement

FIG. 3 illustrates one embodiment of an asymmetric Wi-Fi communicationsystem. Referring to FIG. 3, Wi-Fi sensor node 301 encapsulates sensorreadings in Wi-Fi probe request messages, such as Wi-Fi probe requestmessage 303, and sends the probe request messages to a Wi-Fi accesspoint 302. A probe request is the first message that a Wi-Fi clientbroadcasts to a Wi-Fi infrastructure when it powers up and looks for anaccess point with which to communicate. In this case, Wi-Fi sensor node301 does not have to be capable of two-way communication. Instead ofinitiating a complex exchange of messages that requires two-waycommunication and links a host with a client, as is typical in Wi-Fi,the probe request messages are received and processed by access point302 and any messages from Wi-Fi access point 302 back to sensor node 301are ignored since Wi-Fi sensor node 301 does not have a communicationreceiver. This is advantageous in that ignoring messages allows thesensor node to reduce, and potentially minimize, its time “on the air”,which reduces its energy needs. In one embodiment, the Wi-Fi sensor nodeincludes a Wi-Fi chip that only sends probe request messages and doesnot have wireless functionality to receive messages wirelessly.

Access point 302 processes probe request message 303. In one embodiment,access point 302 extracts the embedded sensor readings data from theprobe request message and creates another message containing the datafrom the sensor readings. Access point 302 sends this message to anotherlocation remote from access point 302. Access point 302 may send themessage via a wired network connection or wirelessly. In one embodiment,access point 302 generates a conventional TCP/IP message 304 and sendsthe message to an IP address via the Internet.

FIG. 4 illustrates one embodiment of an access point, such as accesspoint 302 of FIG. 3. Referring to FIG. 4, access point 302 comprises awired communication interface 405, a first antenna 402 that receives theprobe request message described herein, a radio 403 coupled to firstantenna 402; and a processor 404 coupled to the first antenna 402 andwired communication interface 405, where the processor generates anothermessage (e.g., TCP/IP message, a UDP packet, etc.) with the data fromthe one or more sensor readings in the probe request message and sendsthat message to a destination via a wired network (e.g., the Internet)using the wired communication interface. Alternatively, access point 302transmits the second message (e.g., the TCP/IP message, a UDP packet,etc.) wirelessly using radio 403 or another radio or wirelessfunctionality.

FIG. 5 is a flow diagram of a process for generating and sending amessage that includes sensor readings data from a sensor node. Theprocess is performed by processing logic that may comprise hardware(circuitry, dedicated logic, etc.), software (such as is run on ageneral purpose computer system or a dedicated machine), or acombination of both. In one embodiment, the process is performed by anaccess point.

Referring to FIG. 5, the process begins with processing logic receivinga probe request message wirelessly, where the probe request message hasdata from one or more sensor readings embedded in the probe requestmessage (processing block 501). In response to the probe requestmessage, processing logic creates a second message with the data fromthe one or more sensor readings (processing block 502). In oneembodiment, this is a TCP/IP message. The message could include one ormore UDP packets. Once the message is created, processing logic sendsthe message to a destination via a network (e.g., the Internet). Thedestination may have an address on the Internet or another network. Inone embodiment, the processing logic sends the message using a wiredcommunication link. Such a retrofitted Wi-Fi access point can act as abridge device, specifying a single common source IP address, (e.g. theaccess point IP address). Alternatively, the access point can act as anIP router, pretending to forward IPv6 frames from phantom IPv6-capablesensors. Unique source IPv6 addresses of sensors can be constructed fromunique sensor-specific information. One example would be to constructthe phantom address as “link-local IPv6 address” from the sensor Wi-Firadio's IEEE 802.11 MAC address, following the Modified EUI-64 procedure(see RFC 4291 <http://tools.ietf.org/html/rfc4291>, IP Version 6Addressing Architecture, R. Hinden, S. Deering (February 2006)).

In one embodiment, prior to performing a message exchange, configurationinformation 310 is supplied to Wi-Fi sensor node 301 when the system isinstalled. In one embodiment, configuration information 310 comprisessecurity information necessary to authenticate the transmissions, MACaddresses for the source, and potentially the destination. In oneembodiment, the physical location of Wi-Fi sensor node 301 is alsosupplied. This will be useful for some applications in which sensors areplaced in fixed locations. In one embodiment, configuration information310 also includes the OUI, or organizationally unique identifier, thatis assigned by the Institute of Electrical and Electronics Engineers,Incorporated (IEEE) and identifies the vendor of Wi-Fi sensor node 301,or at least the Wi-Fi chip within it. In one embodiment, configurationinformation 310 also includes an address that identifies the recipientof the sensor readings data. This may be a destination IP address on theInternet. As discussed above, the data that is sent from the Wi-Fisensor node 301 is data from sensor readings.

However, other dynamic data may be included in messages and wirelesslytransmitted. This dynamic data may include timestamps, error codes,battery status, etc.

Channel Contention Resolution

As discussed above, the sensor node includes a monitoring unit (e.g.,monitoring unit 110 of FIG. 1B) to monitor the wireless channel to avoidcongestion caused by other devices transmitting at the same time.Interference on the channel could limit the ability of the access pointto receive the message transmitted by the sensor node.

In one embodiment, one solution for channel contention is to broadcastthe probe request message N times, where N could be small as one. In oneembodiment, the number of transmissions depends on the number of otherclients on the channel. In an environment where there are no other Wi-Fiusers, every transmission should be correctly received. In any case, bystraightforward application of probabilities, out of N messages, where Nis less than five, at least one of them will be received, even if thechannel is heavily congested. This would provide almost guaranteedreception, but at the cost of a linear increase in energy. This is asignificant issue in applications that are severely limited by availableenergy.

In an alternative embodiment, the monitoring unit uses an envelopedetector to avoid channel contention. Instead of a receiver, theenvelope detector monitors the incoming RF energy for a lull in energyand causes the sensor node to broadcast the probe request at thatinstant, during the lull. The lull in energy may be the time the energyis less than a threshold. In essence, the monitoring unit is performinga simplified version of the Wi-Fi Channel Clear Assessment.

There are a number of alternative approaches to determine if the channelis clear that may be used and implemented using the monitoring unit.

In one embodiment, the monitoring unit comprises a logarithmic amplifierdetector (also known as a logamp). This is a device that takes RF inputpower and converts it to a voltage output. A comparator receives theoutput voltage on one input and a pre-determined fixed input level onthe other input. The fixed input level represents the maximum powerlevel indicated as “clear” (such as anything less than −30 dBm), and anytime the voltage output from the logarithmic detector is below thatfixed input level (such as <1V in this example), the channel isconsidered clear, and it is safe for the sensor node to transmit theprobe request message with the embedded data from sensor readings. Thiscomparison arrangement is shown in FIG. 8A.

In another embodiment, the monitoring unit comprises an RF powerdetector. An example of this type of device is the LMH2100 of TexasInstruments. In one embodiment, the output of the RF power detector isdigitized through an analog-to-digital converter (ADC) and compared by amicroprocessor to determine if the incident energy is sufficiently low.The same example numbers as above might apply (for example, if thevoltage is <1V, that may be sufficiently low). However, this has thesame advantages and disadvantages as using logamp.

In yet another alternative embodiment, the monitoring unit includes anRF power detector and an analog comparison as a single part. An exampleof this is the LTC5587 of Linear Technology, which gives a singledigital output that is used to generate an interrupt when the power isbelow a certain threshold (pre-determined and programmed during systemconfiguration).

In still yet another alternative embodiment, the monitoring unitincludes an RF rectifier that rectifies the RF input to create a verysmall voltage. The monitoring unit also includes a low-voltagecomparator/detector that compares the voltage to a threshold voltage.FIG. 6 illustrates an example of an RF rectifier. Referring to FIG. 6,V1 represents the RF voltage input that gets half-wave rectified by D1.R1 and R3 are adjusted to set a level below which the voltage (andincident RF) would be determined to be “clear”. In one embodiment, themonitoring unit considers it safe to send a packet if the output is<0.3V. A comparison arrangement that uses the output of the RF rectifieris shown in FIG. 8B.

In one more embodiment, the monitoring unit includes a voltage detectorthat generates an output that has been amplified by an op amp circuit.The signal above, out of D1, could be input to a calibrated voltagemultiplier op amp, such as, for example, the AD8293 of Analog Devices.This creates an 80× amplified signal based on a low-voltage swing input.The 80× amplified signal is easy for a microcontroller ADC to digitize,and then a determination can be made as to whether the energy in the RFchannel is sufficiently clear to send a package.

FIG. 7 illustrates a voltage detector generating an output that isamplified by an op amp. Referring to FIG. 7, power is measured across avoltage drop resistor and amplified into an ADC. Note that a simpleextension to provide a low-scale rectified voltage from an antenna andamplify this into an ADC for determination of whether energy is presentin the air (channel clear detection) may be used. An example of acomparison arrangement involving these components is shown in FIG. 8C.

In various embodiments of the monitoring unit above, the signal outputcould be used as a hardware control either for an RF switch input or adigital transmitter “enable” pin, so that the transmitter is switched on(or off) in hardware as soon as the state of the channel is determined.Instead, software could begin looping the transmission with the radiogate turned off. As soon as the channel is determined to be cleared byone of the above methods, the hardware enables the radio automaticallyto send the packet with the smallest possible latency.

Alternative Embodiments

There are a number of alternative embodiments. The following describessome of the alternative embodiments.

Custom Transmitter Combined with 802.11

In one embodiment, the transmitter directly synthesizes probe requestmessages. Therefore, the sensor node keeps the data packet, modulation,transmission in the digital realm as long as possible. Often, themodulation is added in the analog realm. But because the sensor node hasforeknowledge of almost the entire packet that the sensor node will send(because it doesn't send any other types of packets), the sensor nodecan pre-construct this packet, and its modulation bits, digitally aheadof time. This creates a transmitter with fewer analog components thanusual, which reduces the amount of power consumption used by the sensornode in transmitting the probe request messages.

FIG. 10 is one embodiment of a transmitter that includes only asynthesizer, combiner, and filter. Thus, the transmitter has a muchsmaller, simpler, cheaper analog section, which is identified in thethick dashed box. In one embodiment, the transmitter uses a shiftregister to implement basic modulation instead of a DSP. In anotherembodiment, the transmitter uses direct digital synthesis of DBPSKmodulation. For more information, see B. R. Jackson, Y. Zheng, C. E.Saavedra, “A CMOS Direct-Digital BPSK Modulator Using an Active Balunand Common-Gate Switches,” IEEE International Symposium on Circuits andSystems, May, 27-30, 2007, 2534-2537. In one embodiment, the transmitteruses scrambler and Barker-11 spreader algorithms implemented in hardware(e.g., a shift register with feedback). In one embodiment, thetransmitter uses a channel-selective filter required to shape thespectrum of direct digital synthesizer to meet the 802.11b spectral maskrequirements (10 MHz or 20 MHz channel).

Fast Startup Time

A significant part of the energy cost to transmit a packet is thestartup time, not only the radio transfer time. In a high bandwidthradio interface, the radio transfer time can be 1-2 usec, but startuptime can be in the milliseconds, and the energy penalty for this can bemuch more significant than the time to merely send the data.

In one embodiment, the sensor node pre-computes or performs operationsto compute portions of the probe request frame quicker. In oneembodiment, the bit fields in the probe request frame are hardcoded andpre-computed with the exception of the bit fields into which sensor datais embedded. Some bit fields are obtained from lookup tables, such asmodulation schemes (encoding) and CRC codes. That is, in one embodiment,the sensor node uses lookup tables instead of computing modulationschemes and/or CRC codes. This can include CRC32 (FCS in 802.11 MAC) andCRC16 (802.11 PHY) checksums. FIG. 11 is a modified version of FIG. 2 inwhich a portion of the probe request frame is pre-computed (seeprocessing block 1101).

In one embodiment, the pre-computed portion or unchangeable portion ofthe probe request frame is cached. Then the CRC is computed over thechangeable portion of the frame, beginning first with the cached value.

In one embodiment, a portion of the probe request packet content ispre-computed. If the sensor node is measuring temperature from 0-70 Cwith 0.5 C accuracy, the lookup table (LUT) has less than 150 rows ofwhich to keep track and the temperature is used as an index into the LUTto find a pre-computed version of the packet content, CRC, encoding, andmodulation. The same type of pre-computing can occur for other types ofsensor data such as pressure or humidity.

In one embodiment, the sensor node is designed to reduce, andpotentially minimize, a portion of the startup time. For example, in oneembodiment, the sensor node includes a MEMS oscillator instead of acrystal oscillator, which stabilizes much faster. In another embodiment,the sensor node performs operations in parallel so that overall startuptime is reduced. For example, sensor nodes starts the PLL stabilizationperiod for the radio transmitter and the sensors read the latest sensordata while the PLL is stabilizing, rather than wait for both events tobe performed in series. In yet another embodiment, the sensor node usesa single power supply rail to provide power from a power supply (e.g.,battery, energy storage, etc.) to the components of the sensor node. Theuse of the single power supply rail avoids the use of power railsequencing (both turning power on and off).

Wi-Fi Collision Avoidance

In one embodiment, standard Wi-Fi protocols are used to ensure that thechannel is clear. In this case, Wi-Fi supports control frames thatcreate a temporary silence on the air. This can be used to guarantee aWi-Fi sensor node transmits a tag in the clear. This is analogous to anemergency vehicle turning on lights and a siren sound, so that theroadway becomes clear, and it can travel quickly and safely to the need.

There are two ways in which to do so. The first involves the Wi-Fisensor node quieting other Wi-Fi devices, and the second involves theWi-Fi access points quieting Wi-Fi devices.

In one embodiment, for the first way, the Wi-Fi sensor quiets otherWi-Fi devices through the use of a “jamming pulse”. In this case, asensor node sends a non-information-carrying part of transmission (“ajamming pulse”) that disrupts Wi-Fi communications and forces all Wi-Fidevices to back off for a particular time interval (as defined in 802.11specification). The sensor node then waits for a minimum back-off timeinterval, as defined in the 802.11 specification, and then begins theinformation-carrying part of transmission (e.g., transmitting the proberequest frame) slightly earlier (1 usec) than a normal Wi-Fi devicewould do it. In other words, the sensor node uses the Wi-Fispecification back-off time to ensure that it gets “clean air” fortransmit. As long as the Wi-Fi packet transmit is in process, otherWi-Fi devices will detect that through their receivers, and will waitfor the sensor node to complete transmission of the Wi-Fi packet.

In another embodiment, in the second way, the access point uses thestandard Wi-Fi protocol to put all Wi-Fi devices into receive-mode onlyfor a certain amount of time. An access point transmits the 802.11control frame which forces all Wi-Fi devices into receive mode forinterval of up to 100 msec. For more information, see the IEEE 802.11Specification. In one embodiment, the periodicity of this signal andsize of the window depends on the estimated network traffic from theWi-Fi sensor nodes. The more of Wi-Fi traffic is observed, the moreoften the access point sends the control frame.

While in receive mode, the channel will be clear for the Wi-Fi sensornode to transmit. In one embodiment, sensor nodes will sense this basedon the channel quality detectors described in this disclosure. In oneembodiment, in the event that there are several Wi-Fi sensor nodes inthe area, their transmit times are randomized based on a uniquehardcoded ID on the sensor node, and timing is determined from when thechannel was cleared by the access point. In one embodiment, the lengthof time for forced-receive-mode is a function of the probe-requesttransmit time multiplied by the anticipated number of sensor nodes inrange. Probe request transmit times should range on the order of 1-100usec, typically, unless the log is very long. Therefore, even if thereare 1000 sensor nodes in the environment, the hold-off time need not begreater than 100 msec.

In one embodiment, the access points in an enterprise are synchronizedand broadcast a command that puts all Wi-Fi devices in receive mode in acoordinated fashion. In one embodiment, knowledge of the physicalarrangement (placement and distance between the access points) is usedto determine a group of access points that should send the quiet commandsimultaneously. For example, if all the access points within 100 metersof a given x-y location broadcast the quiet command at the same time,this would guarantee that the sensor nodes within 50 meters of that x-ylocation would see no energy and thus they could begin transmittingprobe requests.

Wi-Fi Sensor Node Collision Avoidance

If multiple sensor nodes decide to transmit at the same time, acollision occurs. In one embodiment, to avoid this problem and improvethe probability that the Wi-Fi air is “clear” when it wants to send itssensor data, the sensor nodes uses a time-based self-retry technique.Using such a technique ensures that Wi-Fi sensor nodes do not interferewith each other when transmitting.

More specifically, in one embodiment, to minimize chance of collision,each sensor node adds a random delay T and waits. If the clear channelassessment (CCA) circuit reports “busy” before T expires, the sensornode returns to sleep (e.g., reenters the sleep mode); otherwise, thesensor node begins transmission. In one embodiment, the maximum T isless than 802.11-defined wait time (sensor nodes shall get on the airbefore any other Wi-Fi device), and the minimum T depends on the CCAcircuit time constant (i.e., the time lag of the CCA circuit—how longdoes it require to detect if there is energy in the air or not). In oneembodiment, the time T is greater than the time constant of the CCAcircuit, and less than the minimum 802.11 wait-time (thus, the Wi-Fisensor node claims the air space for transmission before other Wi-Fidevices join in).

The time lag of the CCA circuit should be very short. For example,logamp RF energy detectorresponse times are faster than 100 nsec, andWi-Fi standard backoff times are a minimum of 10 usec. Therefore, in oneembodiment, Wi-Fi sensor nodes are randomized, based on hardcoded IDbits, to back-off between 100 nsec and 10 usec, thereby increasing theprobability of “clean air” when they want to transmit. In oneembodiment, the number of randomization slots is adjusted by a sensornode based on Wi-Fi backlog, i.e. how many times the sensor failed tostart transmission.

Whereas many alterations and modifications of the present invention willno doubt become apparent to a person of ordinary skill in the art afterhaving read the foregoing description, it is to be understood that anyparticular embodiment shown and described by way of illustration is inno way intended to be considered limiting. Therefore, references todetails of various embodiments are not intended to limit the scope ofthe claims which in themselves recite only those features regarded asessential to the invention.

We claim:
 1. A method comprising: embedding, by a sensor node, data fromone or more sensor readings in a field of a first message, the fieldbeing designated as part of a standardized protocol to send informationother than the data sensor readings; determining without the use of areceiver, by the sensor node, whether a wireless communication channelis at a predetermined quality level to send the first message; andbroadcasting the first message wireles sly to a component according tothe standardized protocol.
 2. The method defined in claim 1 wherein themessage comprises a probe request message.
 3. The method defined inclaim 1 wherein the standardized protocol is Wi-Fi and the component isa Wi-Fi access point.
 4. The method defined in claim 1 whereindetermining whether a wireless communication channel is at thepredetermined quality level comprises monitoring incoming radiofrequency (RF) signals using an envelope detector.
 5. The method definedin claim 4 wherein the envelope detector comprises a logamp.
 6. Themethod defined in claim 1 wherein determining whether a wirelesscommunication channel is at the predetermined quality level comprisesconverting incoming radio frequency (RF) signals to a voltage andcomparing the voltage to a threshold voltage level, and further whereinbroadcasting the first message is performed if the voltage is less thanthe threshold voltage level.
 7. The method defined in claim 1 whereindetermining whether a wireless communication channel is at thepredetermined quality level comprises: converting incoming radiofrequency (RF) signals to a voltage using an RF power detector;digitizing an output of the RF power detector; and comparing thedigitized output to a value, and further wherein broadcasting the firstmessage is performed if the digitized output is less than the value. 8.The method defined in claim 1 wherein determining whether a wirelesscommunication channel is at the predetermined quality level comprises:generating a digital output in response to performing RF powerdetection; and generating an interrupt responsive to the digital outputindicating the RF power is below a threshold, and further whereinbroadcasting the first message is performed in response to theinterrupt.
 9. The method defined in claim 1 wherein determining whethera wireless communication channel is at the predetermined quality levelcomprises: rectifying incoming radio frequency (RF) signals to create avoltage; comparing the voltage to a threshold, and further whereinbroadcasting the first message is performed if the voltage is less thanthe threshold.
 10. The method defined in claim 1 wherein determiningwhether a wireless communication channel is at the predetermined qualitylevel comprises: monitoring incoming radio frequency (RF) signals usinga voltage detector; creating, using a voltage multiplier op amp, anamplified signal based on a voltage swing input from the voltagedetector; digitizing the amplified signal; and comparing the digitizedamplified signal to a value, and further wherein broadcasting the firstmessage is performed if the digitized output is less than the value. 11.The method defined in claim 1 further comprising: creating, in responseto receiving the first message wirelessly, a second message with thedata from the one or more sensor readings; and sending the secondmessage to a destination via a wired network.
 12. The method defined inclaim 11 wherein the second message is a TCP/IP message in the form of aUDP packet and the wired network is the Internet.
 13. A sensor node foruse in a sensor network, the sensor comprising: one or more sensors forsensing and logging data; an antenna; a transmitter coupled to theantenna to transmit information wireles sly; and a controller coupled tothe energy harvesting unit, the one or more sensors, and thetransmitter, the controller, using energy previously harvested andstored by the energy harvesting and storage unit, to: embed data fromone or more sensor readings from the one or more sensors in a field of afirst message, the field being designated as part of a standardizedprotocol to send information other than the data sensor readings,determine whether a wireless communication channel is at a predeterminedquality level to send the first message, and signal the transmitter tobroadcast the first message wirelessly to a component according to thestandardized protocol.
 14. The sensor node defined in claim 13 whereinthe message comprises a probe request message, the standardized protocolis Wi-Fi, and the component is a Wi-Fi access point.
 15. The sensor nodedefined in claim 13 further comprising: an energy harvesting unitoperable to convert incident energy to direct current (DC); and anenergy storage unit operable to store recovered DC power.
 16. The sensornode defined in claim 13 further comprising a monitoring unit todetermine whether a wireless communication channel is at thepredetermined quality level by monitoring incoming radio frequency (RF)signals, the monitoring unit to indicate to the controller when thewireless communication channel is at the predetermined quality level tocause the controller to signal the transmitter to broadcast the firstmessage.
 17. A method comprising: receiving a probe request messagewirelessly, the probe request message having data from one or moresensor readings embedded in the probe request message; creating, inresponse to receiving the probe request message, a second message withthe data from the one or more sensor readings; and sending the secondmessage to a destination via a wired network.
 18. The method defined inclaim 17 wherein the second message is a TCP/IP message and the wirednetwork is the Internet.
 19. The method defined in claim 17 wherein thefield is designated as part of a standardized protocol to send vendorinformation.
 20. An access point for use in a communication systemhaving at least one sensor node, the access point comprising: a wiredcommunication interface; a first antenna, the first antenna to receive aprobe request message, the probe request message having data from one ormore sensor readings embedded in the probe request message; a radiocoupled to the first antenna; and a processor coupled to the first andthe wired communication interface, the processor to create, in responseto the probe request message being received, a second message with thedata from the one or more sensor readings and to send the second messageto a destination via a wired network using the wired communicationinterface.